In Connecticut, this is covered by the firefighter's rule. Police and fire personnel entering a property as part of their official duties are considered licensees, which limits the duties of the landowner. The rules are as follows:
- You can't intentionally hurt or lay a trap for the licensee.
- If you know or should know the licensee is there, you need to exercise due care with them.
- You don't have to worry about obvious hazards (but keep in mind that it's harder to see stuff at night).
- If you're doing something dangerous, you need to watch out for them.
- If you know about a hidden hazard, you must warn them.
I'm not sure how in-depth you need to go with the warnings; various things I find suggest the duty to warn might only be there when you know or should know the licensee is present, but signs are a good idea regardless. On the other hand, if you do need to warn them, you might need to mention the specific locations of the pits you actually know about. However, there's no duty at all to proactively look for possible hazards.
This rule originated as a rule for professional firefighters responding to a negligently-started fire: the idea is that professional firefighters sign up to do a dangerous job, and letting them sue for hazards inherent in their job (they aren't called without a fire) is a bad idea. Also, since they cannot be denied entry, go in places not open to the public, and can arrive at any hour, needing to keep the property safe for them is an unreasonable burden. Of course, there's an exception if a law is passed to protect their safety, because statutes override common law.
The rule has since been extended in some states to police, and to situations besides the very problem they were called for. Other states have abolished it. In any event, this is for civil liability only: this is when cops can sue for injuries caused to them.