I too have had this problem (see attached photo), and sent my M5 to Sherry back in 2014. When I asked her the cause of the leak, she replied "The housing shell was cracked!" and intimated that it had been poorly worked on by a prior repairer. I'd purchased the camera used, and it looked very good from the outside, so I was surprised that the camera could have been mishandled to the point the housing shell was cracked.
She made a repair (not inexpensive), returned the camera, the problem continued so I returned it to her, and it came back fixed. I found a small errant dab of what looked like a black sealant on the camera exterior, so I presume that she'd used some sort of flexible caulking to eliminate the leak.
The one thing I did that I rather regret was to have her adjust the meter to operate on 1.5V, on the theory that I could find "silver alkaline" 625 batteries (her words in quotes). I wish I'd left it unchanged, so I could continue to use a diode-based adapter with 357 silver button batteries to get a final output of 1.35V. I'm going to try using the adapter/357 again to see what it gives, in the hopes that at least its voltage will not vary as much as the regular alkaline 625 batteries that die a quick death. if anyone has a fix on "silver alkaline" 625 batteries, please let me know.