Of course, turning the mike gain to zero in SSB transmit should get you ZERO on the external wattmeter.
If you do show a steady or fluctuating meter reading, it leaves two possibilities.
1) The SSB mike audio has a buzz, hum or squeal on it, and that's what drives up the wattmeter, spurious audio in the sideband modulator.
2) The driver and/or final stages are oscillating. If there is enough accidental RF feedback in the power sections of the transmitter, it will produce its own, separate RF signal. A frequency counter placed in line would show a reading that is nowhere near the selected channel frequency.
My bets are on option two. This is yet another odd symptom that can be identified at a glance with a spectrum display of some sort. Don't have to have a multi-kilobuck lab instrument, although they're quite nice. More like a 30-dollar RTL USB dongle plugged into your computer running free SDR software. Most of them go down to about 25 MHz without an adapter like the NooElec Ham It Up.
A cheap SDR is not a bad spectrum display.
These guys are a good place to check that out.
https://www.rtl-sdr.com/rtl-sdr-quick-start-guide/
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