My guess is that Bob (the person) doesn't have to necessarily know that there's an auction going in the blockchain, but he can launch a transaction and look for eUTXOs of this type of contract. If the off-chain code finds these specific type of eUTXOs, then he can place a bid.
If you look for in the EnglishAuction.hs file, line 334:
findAuction :: CurrencySymbol
-> TokenName
-> Contract w s Text (TxOutRef, ChainIndexTxOut, AuctionDatum)
findAuction cs tn = do
utxos <- utxosAt $ scriptHashAddress auctionHash
This is the function responsible for this. This happens also when bidding (line 277), or closing the auction (line 309), or with any other endpoint the contract opens to the public.