The Maryland state law authorizing speed cameras states that before activating an unmanned stationary speed monitoring system, the local jurisdiction shall... "Ensure that each sign that designates a school zone indicates that speed monitoring systems are in use in school zones."
The following image is of a school zone sign for a camera at 2700 block of Orleans Street in Baltimore City.

We have received similar reports about the lack of signs indicating speed cameras in use elsewhere in the city.
In addition, according to a citation received by the car's owner (who was not the driver of the vehicle in this case) the camera is apparently issuing tickets based on this 25mph speed limit, and is doing so from 6am-8pm. Preceding this sign the speed limit is posted at 30mph. But look at the sign, it reads "7:00 a.m. to 4:00pm". A driver who read this sign might reasonably believe they were still in a 30mph zone and that the school zone was not in effect after 4pm when school is out of session.
Of course it is unlikely Baltimore will ever bring all of its speed camera zones into compliance with the law. Unless drivers in Maryland voice their objections in the November elections and vote against the lawmakers who approved the cameras you won't need signs to know where the cameras are: they will be everywhere.