proof of documents required at the time of immigration

[John]

If you have an ILR visa you can use it to re-enter the UK ... as a returning resident! So if someone ... not you ... was using their ILR like a glorified visitor's visa then they might encounter problems.

Also do appreciate that ILR is slightly wrongly named, in the sense that that "Indefinite" really does not mean that. That is, if there is a continual absence from the UK exceeding two years then you effectively need permission from a British Embassy/Consulate to use that ILR visa. But any absence under two years and no problem should be encountered.