Use whatever search you have that shows they phoned home.
Use fill Null to set the fields to something like-
Your Search |fillnull value="NoData" clientAStatus field2 field3
http://docs.splunk.com/Documentation/Splunk/6.2.2/SearchReference/Fillnull
Generate alerts when ClientAStatus = NoData.
The NoData Status will only be filled if no the search returns no results in the time frame you specified.
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Quick way to group them:
index=test sourcetype="answers" | transaction maxspan=5s thost
Assuming the whatever is polling them is doing it serially and there is a max span of 5 seconds.
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