Horn Sounding when Start button Pressed HELP!

Been out to the bike, as an aside remove back wheel as it had a slow leak on the new tyre so wanted to check the beads, but it was the new valve that was leaking, had to put a new core in. But it did give me a bit more access to get the underseat panel off to look for the alarm unit. Which wasn't there, the wiring is, but nothing connected.



Put it back together, fetched the battery from the workshop where it had been on charge since last night, connected it up, pressed the starter, it fired & ran. Stopped & started it a few times, been up & down the road all is good, lad is going to take it out for a run tonight. All I can think is it was the battery, but we bypassed the battery last night, took it out of the circuit when we were jumping from a running engine

But thanks for all the suggestions, looks like we'll just have to make sure the Oxford trickle charger is connected properly in future, or turn it over every couple of weeks if it's staying in the garage.

Just a shot in the dark, the ZFE has a permanent live feed which is not disconnected by switching the ignition off.
Perhaps leaving the battery off the bike for a few hours has given the ZFE ecu a chance to re-boot and cure the glitch.
The horn feed is from the ZFE.

i'm with steptoe. the battery is the problem, my son has been out to a few cars with alarms sounding due to discharged main battery during lockdown. there was no real load on your battery with the starter lying on the floor.

Looking at the photo there is no an alarm unit fitted.
Another after thought, load check the continuity of the earth from the ZFE, ( thick brown wire).