No they shouldn't be able to find out, you have to sign a waver about criminal records etc but if you aren't functioning so well or take a lot of time off then their probably going to want an explanation at some point anyway. To be perfectly honest i wouldn't go back to work unless i had been well for a considerable amount of time and had the right balance of meds/good support system etc - of course not everyone has the option of staying off of work. If anything i wish there was more understanding of illness in the workplace whether it be mental or physical - just gotta keep pushing on i suppose...