No 12050, Tuesday 04 Jul 2017, Incognito

Theme words highlghted. I leave it to the commenters to name the theme.

ACROSS
1   I quit Sion in agitation, leading to enquiry (11) INQUISITION*
9   Probably where a sailor is confused on a voyage (2,3) AT SEA [MD]
10 Mistakenly, devil robs sweethearts (9) LOVEBIRDS*
11 Chopper model is in renovated room along with bike (10) MOTORCYCLE {MO{T}OR*}{CYCLE}

12 Toll on a cab (4) TAXI {TAX}{1}
13 Beat extra-terrestrial leaving instrument (5) TRUMP TRUMPet
15 On tower, short person produces sweat (8) PERSPIRE {PERson}{SPIRE}
17 We/I ought to transform and dominate (8) OUTWEIGH*
19 One Scot soldier's patronage (5) AEGIS {AE}{GI'S}
22 Liquor setter found on moor (4) FENI {FEN}{I}
23 S W Europeans left renovated guest-house after throwing out irritated host (10) PORTUGUESE {PORT}{GUESt-hoUsE}*
26 Freed Theodore after Bel-Air massacre (9) LIBERATED {BEL-AIR}*{TED}
27 Indian Navy permitted access to creek (5) INLET {IN}{LET}
28 Await other ranks/soldier to get medicine (11) EXPECTORANT {EXPECT}{OR}{ANT}

DOWN
1   Bad sin by alien in picture in picture (5) INSET {SIN*}{ET}
2   Double! Double! Student is awash in endless money after sports vehicle ... (9) QUADRUPLE {QUAD}{RUP{L}Ee}
3   ... embeds crawling snail around end of alley (6) INLAYS {SNAIL}* around {a..eY}
4   Include a small quantity in damaged vein (7) INVOLVE {VOL} in {VEIN}*
5   For starters, increase confidence building measures ... to build a short long-range weapon (1,1,1,1) ICBM Acrostic
6   Ran back with sailor, telling, ... (9) NARRATING {RAN<=}{RATING}
7   ..."Little Thomas climbed up carrying gun to get a rodent" (6) MARMOT {ARM} in {TOM<=}

8   Hope for a Church's towering structure (6) ASPIRE {A}{SPIRE}
14 Wayward sub-lieutenant takes off crumpled suit... that's not defensible (9) UNTENABLE suB-LiEUTENANt*
16 Violently gnash and rail badly at Hilton's secret haven (7-2) SHANGRI-LA {GNASH}*{RAIL}*
17 Sal's innards? (6) OFFALS {SAL}* [RA]
18 Sailing ... coasting without a person having knowledge (7) GNOSTIC COaSTING*
20 Clammy animal accommodation accommodates Workers’ Educational Association (6) SWEATY {S{WEA}TY}
21 You may be photographed here, Romeo, at ten (6) STUDIO {STUD}{10}
24 Dash, showing energy, and sprinkle talc (5) ECLAT {E}{TALC}*
25 Pack minced prawn after removing tail (4) WRAP PRAWn*

GRID