If they're working 0730-1600, and for example taking 60 minutes for lunch they'd be 'working' for 7 hours 30 each day. Most people work a 7 hour 24 day (37 hours per week), so if they did that 5 days a week, they'd accumulate 30 minutes of [FONT=Tahoma, Calibri, Verdana, Geneva, sans-serif]Flexi-time each week. If they're not taking a full hour for lunch, then that would build up their Flexi a bit quicker.
If they're not working an agreed overtime regime, and are on 37 conditioned hours per week (42 including lunch) then I wouldn't expect them to be building up TOIL, just Flexi.
A lot depends on the individual's circumstances and, more importantly, what has been agreed with their Line Manager (Delivery Manager in DE&S). I can only assume you're not their LM (DM), otherwise you'd have been involved in agreeing any working arrangements.
Before getting into any "heated debate", it'd be worth keeping your powder dry until you have a better understanding of the individual's circumstances, you may have to face the fact that it might just be none of your business. Many people now have other flexible ways of working that they've formalised with their management chain. Also, some people get (honestly) mixed up between what is Flexi or TOIL, they shouldn't but you know how thick people can be.
If you feel strongly enough about it, it might be worth having a quiet word with other team members (even the LM) to get a better understanding of the arrangements that people have made, if only to understand why some people are around at various times and some aren't.... Perhaps couch it in terms of helping you to understand the team better, or so you can help people by taking messages for them if they're not at their desks... You could even really get your tongue deep in your cheek and look at it from an 'equality'/'diversity' personal development angle - if you get my drift.... ;-)
If you believe the person is fiddling.... be careful in the first instance, taking that path without a full, clear and factually accurate knowledge/understanding of the situation can take you into a minefield that gets pretty uncomfortable, pretty quickly - this isn't an 80s squadron crewroom....
Watch your six (and preferably get someone watching it for you) before going all formal
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