The first sign that something is wrong is usually a silence you explain away. Dad did not pick up, so he was in the garden, or the phone was on silent, or he is annoyed with you and making the point. Most of the time that is correct. The trouble is that you cannot tell the ordinary silence from the bad one unless you agreed in advance what a normal day looks like. A contact rule does that job, and it costs nothing to set.
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Pick the contact you already have
Do not build a new system he will resent. Use the call, text, or visit that already happens, and make it a fixed touch at a time he can live with. A short call at the same hour each day works. So does a text he answers with one word. The point is that the touch is predictable, so the absence of it means something. If he refuses a daily call outright, say so plainly in your notes and move to the next option, which is a neighbor or a standing visit, not a longer argument about the phone.
Write down what counts as missed
Decide the rule before you need it. For example: one call, no answer, then a second try an hour or so later, and only then does the gap count as missed. Write that rule in one line and keep it where the family can see it. Dad does not have to agree to the rule for it to exist, but it helps if he knows it. A missed touch under an agreed rule is a fact. A missed touch with no rule is an argument waiting to start.
Name the second call and the drive-over
The gap needs an owner. Name one person who calls after a missed touch, and one person who goes over if the second call also gets nothing. If you are the only name on both lines, the rule will fail on the day you are at work, asleep, or out of range. Two names is the minimum. Put the times and the names in the same message thread so nobody has to ask who is supposed to move.
What to look for when you arrive
Check the things that tell you the difference between a quiet house and a stuck one. Is the car where it usually is. Is the mail stacked up beyond a day or two. Does he answer through the door or the window when you knock and call his name. Is the side door latch working when you test it from outside. If you find him inside and he is fine but embarrassed, you have learned the rule works and the gap was real. Write it down either way. A one-line log with the time, the attempt, and what you found is enough. Over a few weeks that log will show you whether the gaps are random or repeating, and a repeating gap is the thing to take seriously.
When the drive-over finds no answer
If the second person arrives, gets no answer, and cannot see or reach him, do not keep knocking for an hour. Call the emergency number for a welfare check and say what you have found. The layout of the house and the existing keys shape what happens next, and Local Help in St. Louis lists the places to turn when you need a hand with the next step. Do not try to force the door yourself unless the person on the line tells you to.
Your next move tonight
Pick the one contact touch you already have, write the rule in one line, and name the two people who act on a missed touch. Text both of them the times and the rule before you go to bed. Then keep the one-line log starting with tomorrow's first touch, whether it goes through or not.