AC-mains capable SSR (Solid State Relay) with very low leakage current?
Ok, I was able to find an explanation why more or less powerful SSR have noticeable leakage current:


  Leakage current comes from the snubber network. The amount of leakage current (typically 10-15 mA) is dependent on the size capacitor used in the snubber network. Crouzet standard SSRs DO NOT include a resistor-capacitor network in most of its relays, which results in a typical leakage current less than 1mA.


Their GN841340xx series are rated at 0.1mA(rms) leakage current for example.

Another reference to snubbers in SSRs: http://www.electronics-tutorials.ws/io/io_5.html