[600MRG] Signal on with no decode

[Merv Schweigert]

Last nite as I checked the WSPR screen just before hitting the rack,
there was a strong signal on "600"  it was in sync with the WSPR
timing,  was plenty strong to decode,  it looked like about -15 but
nothing at all decoded.
I checked the WSPR active page and no one was reporting as being
on 600.    Anyone know who it was, or perhaps it was another mode
in operation?

73 Merv K9FD/KH6  WH2XCR