Security through Obscurity
Mars is a big place. If nobody is looking, it's unlikely anyone will find it.
The reality is that the surface of mars has largely been mapped topologically, so nobody is likely to run a radar scan to map it again any time soon, and even if they did, nobody is going to go over every mile of it looking for unusually regular shapes like domes.
It's very possible the dome would be faithfully reproduced in imagery and radar mapping by a satellite passing overhead and nobody will notice.
If they did tell a computer to scan through the data looking for hallmarks of artificial structures, it's likely they'd get a lot of false-positives too, there are a lot of big rocks, hills and in some places dunes.
The fact is that if NASA realises there's definitely a base and has any idea what to look for, they'll find it in a few days if they've previously gathered the data, or a few weeks if they haven't.
But in the normal course of things, unless you've been foolish enough to put the dome near any sort of obvious landmark or in the middle of one of the big-name valleys then it's highly unlikely to be found.