Redemptionis Sacramentum addresses this in chapter VI, section 2, item 138:…] Still, the Most Holy Sacrament, when exposed, must never be left unattended even for the briefest space of time. It should therefore be arranged that at least some of the faithful always be present at fixed times, even if they take alternating turns.
When I walk into an Adoration chapel and find it empty save for the exposed Blessed Sacrament, I stay until someone else appears even if it means I’m there much longer than I intended to be. I realize not everyone can do this, and there are times when I can’t either. How I handle the matter when that’s the case depends upon which Adoration chapel I’m in and what time of the day (or night) it is.
After the fact, I bring it to the attention of whoever was responsible for the chapel and/or Adoration just to let them know, and then leave it at that. It might have been a freak occurrence, or it might be a recurring problem that needs to be addressed. Either way, once reported the situation is in the hands of the person(s) capable of taking whatever action is necessary.
Lest anyone get the impression this happens to me all the time, please note that I have seen this situation maybe three times in fifteen years. YMMV.
God bless