What does a platypus bill pick up from a shrimp's muscle activity without relying on water motion?

Answer: Faint electric fields from the shrimp's muscle activity

Chemical scent trails released by the shrimp's body — Chemical scent is not the signal from muscle activity that the bill detects electrically. A diving platypus closes its nostrils, while electroreceptors in its bill register weak fields from prey. Chemical traces and electrical signals are different kinds of information.

Faint electric fields from the shrimp's muscle activity ✓ — Correct. The bill carries electroreceptors that respond to the weak electric fields made by living tissue, so a shrimp's muscle twitches can reveal it even when water movement is confusing. The same bill also has mechanoreceptors for pressure and motion, which is why the platypus can combine two senses rather than depend on flow alone.

Pressure waves produced by the shrimp's movements — Pressure waves require water motion, so they are not the separate signal asked about here. The bill does detect pressure through mechanoreceptors, but its electroreceptors also detect fields generated by muscle activity. Combining these channels helps a platypus locate underwater prey.