simple home monitoring?

[Karen Lewellen]

Hi folks,
I am wondering if there might be a very simple Linux solution for this?
I live in an apartment building.  I am not concerned about entry when I am 
in the space.  the locks  I use make that rather impossible.
However I am interested in a way to monitor if anyone comes into the 
apartment when I am out and cannot use the second inner lock.
There are people on staff who have keys, of course they are not supposed to 
come in when I am gone, but oh well lol.
Would be nice is a simple system that I can dedicate a machine for, 
meaning a basic setup, with few resources required.
perhaps software that would email me an alert  or monitoring results.
ideas?
thanks.
Karen