It is amazing just how many spy / eavesdropping gadgets are being sold these days. Some are very clever. Some are very small. All are easily available and affordable.
For a peek at where Spy Shops shop, visit a Spy Shop Super Distributor. There are many others, but this will give you a good idea of what the average person can buy.
This is why TSCM (bug sweep) services are so necessary, and worthwhile. ~Kevin