A bidirectional TVS diode may exhibit such behaviors; when the applied test voltage is below the breakdown threshold, they will appear to be slightly capacitive with a high leakage resistance; apply a larger test voltage and the breakdown voltage may be observed. Bidirectional devices exhibit such behaviors more or less symmetrically. They are commonly used as protection against static discharge when external signals connect to a board, as appears to be the likely case for placements Z6,7 illustrated above.
The given measurements would appear consistent with an SOD323 package, and a 6.8V breakdown would be a likely choice for a system designed to operate from 5v logic.