There is a daily checks and balances (audits) with the third party designated wallet. In most of the ETF's, that third party is Coinbase. They can however take in money today at 42K BTC and buy it 2 to 4 days later at 39K. That could obviously work against them as well. They are making money on the spreads, not the fees. That's why its a been a race to the bottom on the fee schedule, with many of them offering no fees for 6 months or longer.