Someone might have lifted a pad trying to put a battery in or take one out (I can't tell precisely but it looks like the D2C15 screen printing is missing part of the C, could be nothing but I see that sometimes if enough trace has been lifted). Depending upon what has been ripped off the fix could be as simple as finding an alternative spot to solder a wire to, or it could be a nightmare involving you having to remake a junction or something.
Just pulled apart mine and there does appear to be a via to the other side of the board next to the battery so you might then have to get a wire to that side somehow, fortunately it only appears to be a dual layer/double sided board. I don't know if it also goes to that top row of components at this point.

 

Looking at mine briefly just now (no continuity test or anything) it seems it immediately went to a small via which then went to the other side of the board, nothing else on top. If the via is gone you get to throw a wire down there (maybe cutting into some case to make room). If there is some trace leading to the via left (and there is something above BT1 on your picture) then you might be able to use that (scrape the mask off and solder to it) and just use glue or something to hold the battery down rather than relying on solder joints.
You should be able to do it with a normal iron and some magnification (no need for hot air and all that, though it might help) but if you are in doubt (try doing the same on a junk board) then maybe drag in someone that does repair electronics.