Are we talking Carlo V in Ernani, IGI?  He conceals himself as the conspirators gather to plot his death, while elsewhere he's being elected Holy Roman Emperor.

That's a shame!  You could have come up to London to see ENO's production, which originated in 2000 (when I saw it several times) and was revived in 2004 or thereabouts.  A taut production with bold Velasquez-inspired set and costume designs.

Not exactly a tomb-hider, but a tomb-related hiding is that of Lysiart in Euryanthe, who hides and overhears Eglantine admitting to having stolen from the tomb of Euryanthe's sister, Emma, the ring from which Emma took poison, distraught at the death of her lover Udo on the battlefield.  Lysiart resolves to use this to discredit Euryanthe and her husband (and Lysiart's rival) Adolar at court.

Now I wonder why an opera full of such glorious music is so rarely performed?  

I wonder if Alberich's turning invisible counts as hiding?
Fafner 'hides' in his cave, though I'm not sure the choice of 'dragon' really enables concealment that well. 

Scarpia hides behind a column in Sant'Andrea della Valle to avoid being seen by Tosca.

And there's another moment in Falstaff, where Nanetta and Fenton hide together behind a screen.

I would go as far as saying that the entire opera of HAMLET is a missed opportunity As in "a missed opportunity to hear a better opera instead".  What a woeful accretion of second-rate cliches it is - it goes in my category of "justly neglected works"    Along with Bellini's CAPULETTI I MONTECHI which I heard this evening - is Romeo another of our tomb-hiders??   Please don't tell me I have to listen to that bloody opera again to find out...   Even if Ciofi was outstanding this evening

Let's start at the very beginning - a very good place to start...

Monteverdi's Orfeo is accompanied by loyal shepherds through many of his ordeals - one of whom has to be a similarly good tenor, to sing the "echoes" of Orfeo's songs.