Backwater Valve Access Sleeve Cap Broken... sewer gas smell?
A sewer back water valve should seal tight but needs to be accessible incase they plug up. Silicone should be ok if a bead around the cap, if this valve is close to the plumbing riser it could very well be the source of your bad smell , I would have also sealed nobody wants sewer gas coming in their home. I think you may have found the root cause I was skeptical in the electrical room but not with this find. Good job at tracking it down.