Please identify this 0603 component

[hanakp]

It has 0603 package size, I made photos from all 4 sides. I found it in input stage of a photodetector amplifier (PIN diode transimpedance amplifier). I tried measuring its characteristic with Keithley 6487, but there was negligible current up to 200 V (I didn't want to go higher). It has capacitance of 1.4 pF up to about 30 MHz, then it slowly increases. I think it's polymer ESD protector, or maybe single-layer capacitor. However, I'd like to know exact type, if possible.

Thanks for any tips!