It is difficult (and dangerous) to diagnose a problem like this without seeing the damage at first hand. There are lots of reasons why a tree may become hollow. The process usually begins when the hardwood at the centre of the trunk is exposed as a result of a physical wound or attacks by fungal spores and bacterial diseases. A hollow tree is not necessarily a dangerous tree. It is common for apples and other tree species to compartmentalize (seal off) live wood from dead or diseased wood in order to prevent problems spreading. The live part of the trunk is the cellular layer just beneath the bark. Therefore the tree can happily lose the heartwood without any serious adverse effects. Take a look in spring to see whether the canopy of the tree looks healthy. Do leaves appear on all branches? Does the foliage look healthy? Is the tree flowering more or less than usual? Look out for other signs of disease such as fungal bracts in the autumn. All of these factors will help to indicate any serious changes in the trees health. Obviously if the tree appears unstable or is dropping branches then you will need to assess the likelihood of it causing damage or injury, as there may be legal implications for the tree owner should an accident occur. If necessary, take remedial action and call in a reputable arboriculturalist.