Some time ago, I asked this question. The problem I had was that the pressure in my central heating system was dropping, and I couldn't find the leak in the system. I'm now convinced that I don't have a leak.
I've heard that cold weather can cause heating pressure to drop, and this relates to the overflow pipe from the heating system. So my follow up questions are:
I really hate to say this, but your suspected problem is probably way beyond the capabilities of a DIYer. If you are still having problems, it may be time to call in a pro. I have not seen any good answers to this kind of question here on the Stack. I don't think we really have a HVAC expert here. Bite the bullet, find a really competent tech with the right diagnostic tools to find and fix your problem. What you are losing in comfort, fuel effiency , and potential fuel costs might be more than the bill from a proper repair. You may have an equipment malfunction and only putting off the inevitable.
I found a very useful answer here which put me on the right track. The phrase that the heating engineer we got used was "Pressure Valve" but it sounds like exactly the same this - he suggested as well that it'd be quicker and cheaper to fit an external one.
The short answer is no, nothing outside the house will affect the pressure in the system inside. That said, some parts of the system may be in the garage, for example, and they may affect the pressure. These are:
Expansion tank: if the pressure changes greatly between hot and cold then the pressure vessel isn't absorbing the expansion. The membrane could have a leak. Check the pressure at the tyre valve on the tank is about one bar when the system is depressurised. If it is it is ok. If water comes out of the valve it isn't.
Pressure relief valve: This should blow at three bar but that could occur easily if the expansion vessel is faulty (ie it can't absorb expansion because it is full of water instead of having an air space). If the vessel is ok then it could have a leak of its own and lead to a slow loss of pressure.
Drain cock: you may have a drain cock outside the house, check that the rubber valve part is in good condition. If a drain valve is soldered on with the rubber in place it can be damaged and leak.
In the house, look for:
Radiator valves: If there is a crusting around radiator valves then they may have a slight leak, perhaps in the gland.
Even if there isn't any evident sign, slow leaks can occur through fittings not being fully tight, where the leak is effectively vapour and leaves no sign (though tissue paper can help to capture it if you have a suspect fitting).
Look first at things that have changed recently (extra radiators etc.)
