I've got two ideas for that.
The first one is that having the meatshield try to intentionally trigger the trap (only to find that it does nothing because the line just marks where two different locations are linked by means of portal) would still require noticing the line first. If the odds are low of any intruders spotting the trap that's possibly even better than a portal which disguises itself as an explosive trap.
The second idea is that even if you notice the line but it doesn't appear to do anything when you step over it that might tip you off that it does something secretly but that would still leave you guessing what exactly the secret effect is, especially if you can't manage to disarm or sabotage it.
An important part of this would be that Serini might have been a high-level rogue but she would still have to work with ultimately limited resources. Weighing all her options she went for an approach which both fits the mindset of a rogue and which would require a high-level rogue to bypass (and there's probably many other tricks and challenges on the way, so bypassing the portal doesn't mean you've beaten the dungeon). And I believe it's been stated before that the Stickworld tends towards low-average levels (the imp at least was impressed by Vaarsuvius's level as a Wizard).
In 3.5 Spot checks don't detect traps which are meant to be hidden (so most of them). You need Trapfinding, which is a type of Search check which is unique to rogues (below Weapon Proficiency and Sneak Attack).
Find Traps allows a cleric to mimic a rogue's Trapfinding ability but it doesn't guarantee success, and most likely is not as good as a rogue of the same level (due to them investing skill points into the Search skill). On top of that finding the trap doesn't necessarily allow the cleric to disarm it.
Disarming a trap, similar to finding one, is a unique feature for Rogues where they use a Disable Device check. With magic traps dispel magic can temporarily suppress the trap, although you'd need to roll high enough (bottom of the page.).
EDIT: Correction, you don't need to be a rogue to find simple traps but given that this is an endgame level dungeon this almost definitely falls under the category of a high-level trap which requires a rogue's Trapfinding ability.
In addition, Search is an INT-based skill, which given that clerics are WIS-based means that even if they use Find Traps they don't have a very high ability modifier to help them out. In contrast, I've always found it pretty convenient to have some INT as a rogue given that INT is good for skills and rogues are meant to be skill monkeys.
Hmmm... That would be interesting although at the same time I feel like Haley might have caught that. As it is she seems to believe that it's a trap which is triggered upon stepping over it (resetting itself immediately after). Of course it could be that she's good enough to detect and activate it but not good enough to understand its exact workings... do we know how Haley compares to Serini at this point?