On two occassions during a recent flight my aircraft was approached by two threatening aircraft alerts - but the aircraft never appeared. They were 30 minutes apart and came from 5nm to collision point within a logical period.

My new MRX unit gave clear +250mtrs to - 400mtr altitude signals from 5nm to close and Alert signals but neither myself or my passenger could see either aircraft. We were at 2,500 feet in perfect weather (as most days are in Western Australia)but neither Alerts about 30mins apart resulted in a sighting or collision.

We were about 60nm from Perth airport (the nearest city) and effectively over a very isolated region (no homes or buildings). What could have set the Alerts off so convincingly and so close? There is no doubt if they were aircraft they would have been spotted.

Please advise what could give such false readings as they are most unsettling for both occupants. I really want to rely on the MRX unit but am almost afraid to turn it on.