Mac had never liked hearing 'officer down' via radio, even more so since he got together with one certain officer. He started getting scared, a little bit, if he ever would admit it to himself.
Under any circumstances, if his CSI team were injured, he would be the first one to call. So he would be able to get hold of the whole situation pretty fast, however shocked, upset, worried, angry, etc., he would have an idea as what to expect.
Sometimes it is the same with Don, when he is investigating with his CSIs. However, it's not as common as he would like. In most cases, Don's duty doesn't concern CSI. For example, when he rushes into a drug deal with S.W.A.T, or, when he tries to collect info from CI, or, when he goes to catch some murders who are easily identified nontheless equally dangerous, or, when he gets destress calls and goes to backup other officers for God knows what kind of case... Their relationship is an open secret within limited CSI members, but no one knows in homicide. So, no one would call him if anything goes wrong.
Except when Don is with him or other CSIs, Mac constantly has no idea where in Manhattan his job would lead him to. When there were emergent situations, the names of related officers would be the first to omit on radio. It's not that Mac would act like a over-sentimental character in some 9pm soaps, worrying to death. On the contrary, he believes in Don. He trusts him not only able to do the job but also take care of others along with himself. As their choice of career, although more risks are in the job description, they are better prepared to deal with those dangers. However, when Mac hears 'officer down' via radio, none of these could stop the instant shiver running through his mind, and the suspense when he is not able to confirm if it's Don.
If he had to come up with a reason, he would have blamed it on PTSD - that day, the radio channel went crazy, no one knew for sure what happened, and he lost her like that. So, he is affraid, that one day, Don would leave all of a sudden, without him even noticing, just like Claire.
But Don will never know, because Mac will never tell him something as stupid as this, something doesn't even make sense. He is a Marine. He is the head of NYPD Crime Lab. He is... the one, who will protect New York City with Don, listen to Don's complaints about destroying another suit catching bad guys while gently massage the bruises on his arm.
In fact, Don knows more than Mac thinks. It's not that Mac acts obviously (for God's sake, this is Mac we are talking about, who knows what he's thinking when he puts up the Poker face?), just, from his own experience, in addition to some trivial stuff, if he still had no clue, then he might as well give up his badge and admit that he didn't love Mac so much.
Honestly, Don is deeply relieved for the fact that Mac is the head of NYPD Crime Lab. If you could say that paper work comes with the rank, then compared with his own pile, he is pretty sure that Mac spends most of his time sitting in front of desk. Even though you can't say the lab is 100% safe (he hates terrorists!), it beats chasing armed suspects all around the city. As for collecting evidence at the scene, it comes from Mac's personal sense of responsibility more than position requirements (as long as he is with Mac, that's acceptable).
Don knows Mac is tough (he is a Marine!), but that little bit near-death experiences of him still scared Don to hell. So when Mac touches him with the same feeling, as invisible as it is, he notices.
Don swears to God, whatever happened, he would stand by Mac. He doesn't complain about Mac's over-attention about his techniques of martial arts, physical and shoot (though it's completely unreasonable to ask him to quit hotdog and donuts). Mac wants him safe, and he is damn sure himself that he wants to grow old by Mac's side.
When it doesn't concern CSI, Don either sits in the squad room filling those God-damned reports, or wanders around streets and alleys all over Manhattan, depending on the situation, either walking or driving, carrying rifle or standard issue. Don has colleauges with him when there's a case, so he doesn't always do this. But whenever possible, he texts Mac. Nothing meaningful, usually like 'I hate ladders', 'This city's full of nutsjob', 'An extra for laundry next month'... Occasionally, some black-humor comes up. Basically, he just texts the first sentence crossing his mind. Mac never replies, and he doesn't expect any. Both of them know how badly it could go when a cellphone rang unnecessarily in the field. If Don really needed response, he would call.
In some extreme cases, Don goes blank, texting something as stupid as 'The weather is lovely', 'There is a rat on the ground'. There was one time that he sent a '!' to Mac. Whenever it happens, Don feels embarrasing and regrets for several seconds, then he lets it go. It doesn't matter what he texts, what he wants Mac to know, is always - I will never leave you the way Claire did.
They are going to tease the stupid father out of Danny, scare Adam for fun, kick bad guys' asses, protect New York City, and then, grow old together. One day, when Don's teeth are too few to even bite a donut, he will still act like he's going to eat one and watch Mac shaking head, and smiling, 'Only one, Don.'