Covering up a weird scenario!

So I work in a building and there a couple businesses in the building. My business is in the basement level of the building. The 2nd and 3rd floors belong to one business as well. We have a connecting stairwell in the back. So I can pretty much take the stairwell up. There is roof access on the 4th floor (just a roof) and I decided to go up to the top and go to the roof. I thought I knew the code for the pin-pad on the roof door but the alarm went off when I opened the door. So I ran :)

Anyway, someone from the other business might have seen me. I could potentially be fired for this but I just thought as long as I deny it, I cannot get in trouble.

Key factors in this scenario are:
a: There are no cameras in the back stairwell.
b: I did not enter through the front door of my business. (Where there is surveillance)
c: People from the other business use the stairwell to exit and smoke outside.

The only way this could be pinned on me is the fact that I am the only one from my business with a key to the back door. I have thought of a way to cover this up. Any advice would be helpful though.