Last week, when ground running my Volution, the rubber bung in the top of one of the frame tubes came out and was sucked into the prop at high revs. Whilst an unlikely occurrence, it is worth checking them as part of the preflight. Having said that, there's not much to check as they are push fit; mine were both in before the engine was started. Having spoken with Parajet, they have heard of this happening on a previous occasion so there may other times that have gone unreported. Their suggestion is to drill a hole and rivet them in, glue them in or remove them completely. Simon W has suggested using some silicone which is probably what I'll do.