So this is some existing device? What's the rest of it look like?
Do you know that the device was not tested for ESD (or to a lower acceptance criteria than you are apparently looking for) originally?
Is the coax prone to ESD strikes to the inner conductor? -- if so, it sounds like a major wiring fault (in particular, defective shielding), more than an ESD problem necessarily. For sure, not something you want to have, with 172MHz leaking everywhere.
Also, so this is just a receiver? Well, switch, I suppose it could be both...
The PTC is in a strange place, but doesn't seem like it would necessarily have much effect. If its resistance is low, it won't affect insertion loss much, and if it's a small chip component, the layout around it can be adjusted to give reasonably stable 50 ohms. Even without such care, it should be less than a pF, nothing significant here. The next most likely explanation is you were measuring the diode junction capacitance, at whatever bias your impedance meter's excitation drove it up to (which probably isn't much, that type is quite slow -- more of a PIN diode than a rectifier; which could be an explanation for its placement, but no bias resistors or other switching components are shown).
Tim