How did I hear this?

You are inside the XXXXXXX. The NecroTech logo glows gently above the front desk, and doors open onto a number of brightly-lit laboratories and computer rooms. The building has been very heavily barricaded.

A portable generator has been set up here. It is running.

Somebody has spraypainted onto a wall.
Since your last turn:

You heard a muffled groaning from inside the building. (41 minutes ago)

Now, there were no bodies in the building I was aware of, and if there were, wouldnt they have stood up first? So, how did I hear a Zed in the building that was caded, as above?

It is possible that a zombie was in the building, but had been idle so long that they had disappeared from the game, logged back in, moaned and moved on.