This can be a frequent problem for many cats and may not always happen when you are around to witness it. For example, many intruder cats may come in to the home via the cat flap (or even an open window) late at night or in the early hours of the morning when you are probably in bed.
Unfamiliar cats coming into the home (whether you encourage them to come in or not) can be very stressful for your cat, because she may feel as though she has to be constantly on the look-out, ready to defend her core territory at any minute. Especially if intruder cats are not vaccinated, they may also spread disease to your cat when they come into your home.
The best way to manage this problem is to ensure you feed your cat far away from the cat flap (so that intruder cats are not enticed by the smell of nearby food) and to invest in a microchip-operated cat flap, so that only cats with the correct corresponding microchip (this is the identification microchip that is implanted under the skin on your cat’s neck) can access the cat flap.