After 10 years I would replace the rubber gasket battery door seal.
The silicone grease is a good idea.
It helps the rubber seal seat properly.
If you think the rubber seal is still good then try this.
Remove the batterys!
Put the battery door back on.
Fill your bathtub with water and hold the detector underwater.
See and bubbles coming from around any seals?
If not leave it in for 5 minutes then take it out and wipe it dry.
Open the battery box and see if any water got in.
If not you should be good to go.
Keep in mind if you send it to Fisher they will not work on it unless it is updated to a CZ-21.
This is not a bad thing as the cost is around $300 but you get back a detector that has a new faceplate, a new case, plus a tune up, and a bit of a warranty.
Go look what a new CZ-21 sells for.
Willee