Remember that sidewalk strips are often technically and legally is still the property of the owner. Easements are provided to allow specific uses (like utilities) the ability to do their business. If you're talking about a strip of lawn in front of someone's house, it very likely isn't an easement of type you can detect without permission. It's their property that aesthetically (and often for safety reasons) separates the road from the pedestrian side-walk. Even the side-walk is the person's property (just ask anyone sued for someone falling on it). And, just because we have the accessibility to walk across it, doesn't mean we can dig in it.
The exception to this would be on public lands/parks property, where specific guidance is provided. For example, my city allows detecting on all city property within a specific set of guidelines (no trowels, ok to use an 8" probe less than 1/4 wide. Cut flaps must have a side still attached to the lawn (true flap, no plug allowed).) As long as we're following these rules, MD'ers can do public sidewalk strips.
Definitely NOT ok for subdivision strips though, that's private property, and requires permission.
I'm just using this as an example, of course, but simply put, just because a sidewalk strip exists, is not a guarantee it is public property... and easements are defined for specific use (utility, fencing, access, etc.).
Skippy
-Jerry