What is this odd large sign near Aalborg airport/millitary airport?
It's a maritime navigation aid. I'm fairly rusty, but I'm pretty sure it's a form of lead mark. When seen from the water, as you travel towards the mark, if you keep the diamond centred between the circled dots, it will have you travelling in a straight line down the centre/safest path of a channel.

It will normally be associated with a channel through shallow water, or a hazardous bend, or promontory, or breakwater, or a safe entrance or exit course for a harbour, or similar. Sometimes there may be a progression of such marks to enable zigzagging along an irregular waterway with a minimum of course changes.