G0OQK by pure tropo?

The most recent transmission from Nick G0OQK (98km) was at 1854z. Unlike earlier transmissions this was received as a single straight line on the WSPR software screen with not even a hint of aircraft reflections. I wonder if this was pure tropo?  His signal strength was -20dB S/N.

The alternative explanation is this was just a single plane flying in such a direction that there was no Doppler on the signal. I think this means a plane exactly crossing the path between us. I need to dig out that software G3WKW mentioned to see when planes are optimally placed. A reminder it was http://www.airscout.eu.