Floor 100 was empty: triggering Mr. Qi’s visit
There are two depth milestones and two different rewards. The first trip to 25 opens the path to finding a note; the special encounter at 100 requires actually reading it.
Finish the earlier depth challenge
Enter Skull Cavern once and check the farm mailbox the following morning for Qi’s Challenge. Reach at least floor 25 in the desert cavern. If you happen to do it on the first visit, that progress can satisfy the challenge without another run. The money associated with this milestone arrives by mail. Do not keep diving solely to find Secret Note #10 in a chest on floor 25.
Check the note itself before making the long run
Secret Note #10 becomes obtainable after the floor-25 challenge. For ordinary secret notes, obtain the Magnifying Glass from the winter shadow-figure quest. Follow the footprints to the bush near the playground and interact with it. When a note drops in normal play, use it from inventory to read it. Before spending a stack of stairs, open the secret-note collection and confirm that number 10 is actually readable there.
Choose the route for the reward you want
After reading the note, reach 100 again during one run. A shaft from above cannot make you miss it by overshooting 100. Using more than ten placed staircases changes Qi’s comment, not the permanent 25 maximum-health increase. If your goal is his honorable dialogue, count placed staircases and stop at ten; naturally discovered ladders and shafts are a separate matter. Look for the meeting and the drink rather than another cash payment.