I just had it happen again at that copper node before I started mining but this time I saw the skeleton go chasing after a dwarf that was near the burial chamber. I decided to really scour the area again and I did find an active tower with 3 skeletons but it seemed pretty far for random wandering skeletons. I finished them off and was thinking 3 skeletons was all I've ever seen at a single tower (and those and the burial chamber external guards have never respawned in my 1000 game days in this same play through). As I went back toward my base I got near another burial chamber I had previously cleared - and who should run up for some sword play? Another skeleton guard.

So I'm thinking at least one of the external guards for burial chambers now respawn - not enough evidence yet for the tower guards.