Yes, it was very likely CoE signage. Same thing as on many other lakes/navigable waterways, especially around dams.
Decide for yourself how much to worry about it, versus keeping yourself safe. Best of my knowledge any CoE employees you'll see there are inspection/maintenence folks; while they may dress like park rangers they most definitely aren't law enforcement officials. Best I can tell from digging around on the computer, CoE doesn't have LEOs of their own. If they stumbled across terrorists sabotaging a dam--and lived to tell about it--they'd have to call in the nearest available LE agency for help.Same deal in the event they spotted a gun toter like one of us. Most likely that would be a local, county mountie, or statie--who wouldn't be much help enforcing the CoE firearm prohibition--but hopefully would do something about a bomb-wielding sabateur.
Interesting side note: There is a public shooting range located near the lake, as memory serves it is operated by...the U.S. Army Corps of Engineers. (No, they didn't erect 'No Firearms Allowed' signage there! ) It's pretty much a self-service, bring-your-own-everything range, just fyi.