Appscript to poll your email for a message indicating a check in. Read the comments.

appscript_checkup.js

/*
 * Purpose:
 *   To check up on you, in case you have a pet or are travelling.
 * 
 * How it works:
 *   - You will add a calendar invite every day or more than that 
 *     with a link to send a email to yourself with a sentinel 
 *     string (i.e. `TAG` below).
 *   - This script will periodically check your inbox for a message
 *.    with `TAG`, if it doesn't an email will go to a set of
 *     recipients below, i.e. `ALARM_MESSAGE_EMAIL_ADDRESSES`
 * 
 * Installation:
 *.  - Fill in the TODOs below
 *   - Create a trigger to run this script periodically -- say 15 minutes. 
 *     Every time it runs it will check your inbox for a message that has `
 *     TAG` in it.
 *   - Create a calendar invite twice a day to email yourself with a 
 *     link like the following:
 * 
 *        mailto:<your-email>@gmail.com?subject=<TAG>
 * 
 *     So, for Jeff with `TAG` of `STELLA_CHECK` it would be:
 * 
 *        mailto:jeffpalm@gmail.com?subject=STELLA_CHECK
 */

// ------------------------------------------------------------------------------------------
// Constants: Configure these values. Each constant's name should describe what it's used for.
// ------------------------------------------------------------------------------------------

/** The number of hours that is acceptable from your last check in. */
const ALARM_THRESHOLD_HOURS = 15;

/** The subject of the email that is sent when a check in fails. */
const ALARM_MESSAGE_SUBJECT = 'Please check on TODO';

/** The body of the email that is sent when a check in fails. */
const ALARM_MESSAGE_TO_SEND = 'Please check on TODO';

/** The recipient addresses of the email that is sent when a check in fails. */
const ALARM_MESSAGE_EMAIL_ADDRESSES = [
  'TODO+TODO@gmail.com',
];
const TAG = 'TODO';

// ------------------------------------------------------------------------------------------
// Don't change anything below this.
// ------------------------------------------------------------------------------------------

function checkEmail() {
  let threads = GmailApp.search('subject:"' + TAG + '"');
  let minDiffHours = ALARM_THRESHOLD_HOURS + 1;
  threads.forEach(t => {
    let msgs = t.getMessages();
    msgs.forEach(m => {
      let date = m.getDate(),
        diffMillis = Math.abs(new Date() - date),
        diffHours = diffMillis / 60 / 60 / 1000;
      Logger.log('date: ' + date + 'minDiffHours: ' + minDiffHours + ' diffHours: ' + diffHours);
      minDiffHours = Math.min(diffHours, minDiffHours)
    });
  });
  Logger.log('Final: minDiffHours=' + minDiffHours + ' ALARM_THRESHOLD_HOURS:' + ALARM_THRESHOLD_HOURS);
  if (minDiffHours > ALARM_THRESHOLD_HOURS) {
    Logger.log('raising alarm');
    raiseAlarm();
  } else {
    Logger.log('no alarm');
  }
}


function raiseAlarm() {
  ALARM_MESSAGE_EMAIL_ADDRESSES.forEach(addr => {
    GmailApp.sendEmail(addr, ALARM_MESSAGE_SUBJECT, ALARM_MESSAGE_TO_SEND);
  });
}