Any detector, and UW camera, anything that is subject to Ozone, or Sun needs to be serviced on a regular basis. NOW,, add saltwater, silt, sand, contaminants,,,,make the service interval sooner than later.
If you don't know how to service the orings yourself, have the factory do it on a regular basis.
I assume the case for the Tesoro is made by IKELITE.
The inner plastic case needs to be PRISTINE/Surgical clean, just as the oring on the cover and the oring itself. After cleaning with multiple soft towels. Use a clean qtip and clean the tracks, oring mating surfaces again. Then, with clean dry hands add a dab of pure silicone to the surface of the oring and work it all around. Not too much. FEEEEEEEEEEEL for any bit of grit with your fingers.( Clean again, and start again if needed.) You have to take your time if you don't want your detector to get flooded and damaged.
I have been diving for 25 years. I am quite familiar with orings, connectors, etc. I have several UW cameras and IKELITE products.