FX Dreamline Leaking off while in storage

I have a .25 Dreamline with the synthetic stock I purchased from an AGN member last year. I noticed when taking it out of the vault yesterday that it seemed to be down about 100 bar, as I always put every air rifle away fully charged.

Filled it, shot it, refilled it. Checked it today and it had dropped from where I had it filled to yesterday.

Where do I start on this rifle to find the leak? Should I send it to someone to reseal it or are these relatively easy for one to diagnose and seal?

Thanks!