They can be very clever at camouflaging a nest once the process is completed.

There are a couple of things that you can do: you can watch her every afternoon to see if you can catch her in the act of nesting. You can also weigh her daily; if she drops a noteworthy amount of weight from one day to the next, that may be an indication that she's laid eggs somewhere. You'll need a fairly accurate scale for that, but any electronic kitchen scale would work.