As the previous posters have stated, you are still bound by the requirements of the spell. It's simply always active. So if the wizard walks into a bar, without concentrating he'll know whether or not there's magic in the bar. If he then takes the time to concentrate, he can discern the strengths and locations of the magical auras. And, as already stated, the fact that adventurers are usually dripping with magic means that the "magic indicator" will almost always be positive.
So in the end, he'll rarely ever get any useful information unless he spends time concentrating.
The spell is fairly easy to get rid of. Start throwing sorcerers with Dispel Magic at the PC, and have them fire off multiple Dispel Magics. You'll get it eventually.
Edit to add: or you can simply kill the PC.