I've just replaced my smoke detector. It was manufactured in 2006 by BRK. This is an ionization-style detector. I took it apart and the track side of the circuit board is covered with a thick waxy substance. It's clearly part of the manufacturing process, and not a by product of nesting spiders. It's 1 - 2 mm thick and fairly uneven, translucent and soft. You can scrape it away with finger nail pressure. Applying a match flame to it causes the substance to melt and drip off just like candle wax. It's not flammable in it's liquid phase.
For all intents and purposes, it looks and behaves just like candle wax. Can we still really be using candle wax to protect pcbs this century? And if so, why?