Neighbour wants to check for flooding

Did you check for potential flooding though? If its genuine you could knock out the electricity below. Can happen more often than people think.

Leaving aside the peculiarity of knocking on your door at 1am, if water is flowing down his walls from the apartment above, then your apartment is the likely source and him calling a plumber is useless unless he has access to your apartment to locate the leak. Surely it is also in your best interest to repair a leaking pipe as your floors will be damp causing damage to carpets/wood.

If the water is coming from your apartment that is where the plumber needs to be.

It could be a leaking fitting, a shower tray not sealed correctly, a faulty appliance etc. etc.

An experienced plumber will be able to find it, access to both apartments may be necessary to trace the source.

His approach at 1am shows he is not a very tactful person.

If you are alone in the apartment could you get someone to come over and be there when the plumber want's access?

Many years ago, the apartment I lived in was both on the receiving end, and cause of leaks from the apartment above and to the apartment below. I very much appreciated the quick response from the owner above once we told him water was pissing down trough the light fittings on our ceiling, and when a leak in out cistern caused part of the ceiling below to collapse, we took rapid action to close the valve and stop the water.

If the source is the op’s apartment, it should be easy to locate, I’d be pretty pissed off if I was told I’d have to wait for someone in the MC to organise a plumber before leak was dealt with when a simple closing of a valve may be all that is necessary. Not much neighbourly goodwill going on here.

One of the main causes of leaks is degradation of the silicone around shower enclosures. You'd never notice it in the shower itself.

But also most apartment blocks have concrete floors, hence the chances that water has run along the floor and dropped into the ceiling below.

A call to management company is the priority - that what your management fees are for.

You are right to be suspicious of this guy. He seems a bit strange. Ask him to call a plumber and pay for the call out and you will let that plumber in.

Maybe phone the management company and ask them to send a plumber to him and if that plumber decides he needs to look in your place then allow the plumber in.

But letting a stranger who acts like this guy into your house, never mind letting him root arount or even try to fix a leak would be a big no no from me.

Imagine the OP does as you say and lets the total stranger into his/her apartment. What happens next?

If the leak is under the floorboards or behind a wall, then the leak can only be detected by... that's right, lifting the floorboards or cutting holes in the wall.

So again, what is the point of letting this guy in?

The guy lives in the apartment below, the op may not know him, but he isn’t a TOTAL stranger. Depending on where the leak is, no, you don’t always need to left floorboards. It could be a sink, bath, toilet, radiator pipe, none of which would involve lifting boards. Also, if you know what to look for, you can identify leaks under a carpet/wood floor without actually tearing them up, then you just close the valve to that pipe to stop the leak until a plumber can get there.

So a little bit of lateral thinking can go a long way.

Right. Except if the leak was happening in the OP's apartment from a toilet or whatever, he'd have noticed them himself. It's pretty obvious that wherever the leak is coming from, it won't be easily found.

And you want the OP to just take this stranger's word for it that he knows what to look for??!

OP was absolutely right, and now he/she is the one sorting it out for this other randomer? OP has gone above and beyond here.

So OP,

What was the outcome of this story?

Did the management company send a plumber to do an inspection?

And where did they discover the leak??