Why does redis remove key before ttl expires?

Question

Redis

1) I write a key with ttl 86400 (24h) to redis (using software).
2) I go into redis-cli and call ttl key - everything is displayed correctly 86k+. I check after 15 minutes - everything is also correctly counted.
3) I go in an hour - the key is deleted. ttl obviously couldn't expire. What could be the problem?