The mariachi band stood where they always stood - on top of the bridge in the second scene of El Rio Del Tiempo. They saw everyone. And most saw them too.
When Epcot closed for the day, they'd have fun with the other animatronics, but being bandmates, the guitarist, the trumpeter, the violinist, and the bassist preferred to stick together.
In fact, sometimes they literally stuck together. When their bones hooked to each other and it took the other bandmates, sometimes the other animatronics, to free them.
After playing together for so long, they'd become more than friends. They were brothers.
A/N
Though El Rio Del Tiempo is long gone now - in 2007, actually - the skeleton mariachi band are still playing on in its successor attraction, The Gran Fiesta Tour Starring The Three Caballeros.