My thought is that she wasn't trying to keep the information from being found - she was trying to keep the compulsion from being found. What if some Aes Sedai happened to stumble upon her tampering, or someone got curious about why people were acting differently? If there were enough compelled people left alive, or messengers, or even people who had witnessed something that they weren't supposed to, they could start to piece things together. It is unlikely that she would be discovered, but it is conceivably possible that this information could compromise her security. It would then make sense to have someone stick around and 'take care' of any problems that arose.