A few possible sources
The piping itself - somewhere a faulty connection or fitting or valve (if this unit cools with water)
Condensation on the piping that isn't directly above the drain pan
Piping and FCU isn't insulated, and it's in an unconditioned (humid) environment that is condensing on its cold surface
The drain pan isn't draining and overflows - could be clogged condensate drain pipe, or you condensate drain piping isn't sloped properly, or the pan isn't sloped
The FCU casing is compromised and you are sucking humid attic or plenum air into the unit and it is condensing on the colder parts of the equipment not above the pan
You are running airflow through the fan coil above 500-600 fpm. The manufacturer has limitations that if you exceed them you'll carry moisture off of the cooling coil and overshoot the drain pan
As for your BMS, it really depends on what your BMS currently monitors, or if you are looking to add monitoring. Not many of the parameters above can really be monitored by a BMS. You can add a moisture sensor to the higher parts of the drain pan which can indicate when it is starting to overflow. You can add a secondary drain pan (code required a lot of the time) that covers the whole footprint of the FCU from below and put a moisture sensor in that. If you have air measuring station in the FCU that can tell you when air velocity is too high - but you usually won't have that for FCU.
