Even if the horn and power sounder are unplugged the indicators will still flash for a short while. The most likely cause is that the alarm horn (under the drivers side scuttle) has suffered the water bug of rust and it has seized! That is all it was on mine and a cheap horn from haflards at 6 quid solved the problem.
The power sounder only works when the main battery is disconnected and should not sound with the main alarm. All the power sounder is is a backup system should any idiot try to remove the main battery to stop the main alarm going off when they try to burgle the car. It senses that the battery has been disconnected and so activates itself. Problem is that the power sounders own internal batteries, which are not replacable because they are no longer manufactured or available to buy anywhere, give up the ghost and so makes the power sounder a useless item anyway. many oofers have simply unplugged it with no detrimental effects. As long as the horn that sits next to it is connected you will be ok. It goes off when the bonnet is lifted anyway so I cannot see the point of both. How the hell do you take the battery off without opening the bonnet anyway? 
Hope that lot of waffle helps you understand a bit better. 
I think so, and is certainly worth looking at. A pity I had the scuttle off this morning, but didn't check the horn, only the pwer sounder, as I understood that they went off together so assumed that any fault would show up on both. The horn looked OK, but I will check it out now. I checked the voltages at the power sounder plug terminals which were:
Car unlocked and locked with bonnet switch made.
1 =0v
2 [ch8776] 6v
3 [ch8776] 6v
4 = -0.4v
No 12v or even close. :-?
Edit: I forgot to say that the bonnet switch plug was disconnected, but, as Rob almost said, reconnecting it made no difference.