Energy Kinketics EKF1 Plate Heat Exchanger - Slow Leak, Can't Pinpoint Source

I have an Energy Kinetics EKF1 oil boiler, installed about 18 months ago. I've noticed blue-green corrosion/staining building up on the copper pipe stubs below the plate heat exchanger (both the "Domestic Out" and "Boiler In" connections), suggesting a slow leak somewhere in that area.

I ran a flashlight/paper towel test with the boiler firing and a hot water tap open to get flow through the exchanger, wrapped paper towels around the union nuts and the 90° elbow, and let it run for ~15 min. Results were inconclusive — there's some dampness but I can't tell if it's coming from the union joints, the elbow fitting, or the plate exchanger itself weeping from the side.

A couple photos attached showing the staining pattern.

Questions:

• Any tips for better isolating which joint/component is the actual source vs. just catching drips from above?
• At 18 months, is this likely a union seal/gasket issue, or should I be thinking heat exchanger replacement already?
• Anything specific to check given it's still under warranty?

Thanks for any input.