Pilot reported inadvertently taxiing into a restricted ramp area at STJ and suggested that clear surface markings and lights should be put in place to prevent other similar incursions.

Narrative

This was our first time into STJ airport. We were unfamiliar with the situation regarding the joint use by civilian traffic and the Air National Guard (ANG). We looked at the taxi diagram en route; and located where the Foreflight app said the FBO was located. Foreflight geo-tagged the FBO to be co-located at the Control Tower/Terminal building; immediately north and touching the ANG ramp. This turned out to be incorrect information. The FBO is actually located further north on the airport; adjacent to taxiway C2. Upon landing and switching to ground; we were given the vague instruction to 'Taxi to FBO via Alpha'. No mention was given to any particular way to exit Alpha and proceed into the ramp area for the FBO. I quickly looked again at the taxi diagram; and saw that the most direct way to get to the (incorrectly labeled) FBO location was to take the right turn at the end of the ANG ramp and arrive at the FBO location marked on Foreflight. I didn't feel comfortable taking taxiway Charlie 3 because the clearance only specified Taxiway A. Looking back; my brain interpreted that as Ground telling us to turn in on the unlabeled ANG RAMP; instead of proceeding past the incorrectly labeled FBO location and entering the ramp at Charlie 3. Approaching from Taxiway A; there were no markings; paint; or lights signifying that this ramp space was closed to General Aviation Traffic. Additionally; we were well past any military aircraft; and didn't get within probably 100 yards of any of them. It was only as we turned North from what turned out to be the ANG Ramp; and into the area in front of the Terminal/Control Tower building; that I saw a red line and lights facing North that would stop traffic from going back the way we had just come from. But there were no such markings deterring traffic coming in from the main Taxiway A. This was my first indication that anything might be wrong. At the same time; it was dark; so this was the first time we could tell that there didn't appear to be any line personnel on the ramp in front of the mis-labeled FBO location. We called Ground; to confirm where exactly this FBO was located. This was the first time he apparently had noticed our taxi path. He sounded surprised; stuttering something like 'Oh you cut through that way'; then recovered and proceeded to direct us further North to the actual FBO location. I do not believe he was watching us up until that point; which is strange because I believe we were the only traffic on both Tower and Ground at the time. A minute or two later we arrived at the actual FBO; and we turned around to face South before engine shut down. It was then that I saw two ANG Security vehicles stopped at the edge of their ramp; facing toward us with lights on. It began to dawn on me at that point that the ramp we clipped the corner of might have been off limits. Things that could have prevented this incident:-on the taxi diagram there is a hatched line around the edge of the ramp. It did not occur to us what it meant until after the fact. We believed this to be a fully GA airport; so didn't suspect any sort of Restricted areas. We later found a small note at the very bottom of the taxi chart; but we had the chart zoomed in for greater detail and to follow along our route and couldn't see the bottom note. A more clear marking on the chart would help to alert GA flight-crews.-Ground control and FBO personnel later each mentioned that this happens 'all the time'. They even shared how a small plane had been recently stopped by ANG Security; and all occupants removed from the plane at gunpoint when they accidentally taxied onto that ramp. With this knowledge of continued pilot confusion; Ground Control should issue clear; more detailed taxi instructions. An instruction to 'Taxi to the FBO via Alpha; Charlie 2' would have prevented all of this. Further; Ground should carefully watch taxiing aircraft to ensure they don't turn in too early and cut across the ANG ramp. Or at least ask if they are familiar with the restricted ANG ramp. That would've gotten our attention enough to prevent the incursion. -I later informed Ground Control and the FBO that the geo-location for the FBO is wrong and lead us to turn in at the ANG ramp to park at the Terminal/Control Tower building. Neither seemed concerned; but I believe it should be corrected immediately to prevent further confusion. -There needs to be clear and ominous paint markings and lights between Taxiway A and the ANG ramp to keep GA pilots away. A combination of red painted lines; perhaps big red letters on the ramp that say 'RESTRICTED'; and even some red in pavement lighting facing taxiway A would prevent the vast majority of these apparently ongoing incursions.