If you have this phone number set up with Google, and if you have 2-factor auth set up, they will do this every time you login from a previous unauthorized machine. Wherever they're trying to log in, there will be a gate where they have to enter teh 6-digit PIN sent to your phone.
If you don't have 2-factor auth set up, I think they will still sometimes do it with new machines or login attempts that may look suspicious (e.g. brute force password attempts).
Thus, if google does have this number for you, it's very likely that someone is trying to log into your account (it sounds like, possibly your wife). The bad news is, they got your password entered correctly (or it was cached somewhere). The good news is, it's very UN-likely that that person will be able to get the PIN and proceed beyond the gate.
If google doesn't have this number for you, it's possible someone else provided it to them, accidentally or otherwise.
If you don't have 2-factor auth set up, I think they will still sometimes do it with new machines or login attempts that may look suspicious (e.g. brute force password attempts).
Thus, if google does have this number for you, it's very likely that someone is trying to log into your account (it sounds like, possibly your wife). The bad news is, they got your password entered correctly (or it was cached somewhere). The good news is, it's very UN-likely that that person will be able to get the PIN and proceed beyond the gate.
If google doesn't have this number for you, it's possible someone else provided it to them, accidentally or otherwise.
If you don't have 2-factor auth set up, go here and do it. https://www.google.com/settings/u/1/security?hl=en